PROTEINS AND THEIR CLASSIFICATION. 913 



N 17 O 6 . The molecule contains no sulphur and is characterized also by its 

 large percentage of nitrogen. Protamin must be regarded as the simplest 

 form of protein occurring normally in the animal body, a protein in which 

 many of the groupings, such as cystin, tyrosin, carbohydrates, found in the 

 usual protein molecule are entirely lacking and in which the basic groupings 

 (arginin) predominate. The histons form a series of compounds intermediate 

 in many ways between the protamins and the usual proteins. The reaction 

 usually considered as characteristic of the class is that they are precipitated 

 by ammonia. They are precipitated also by the alkaloidal reagents e. g., 

 phosphotungstic acid in neutral solutions. Ordinary proteins give a pre- 

 cipitate with these reagents only in acid solutions, while the protamins give 

 one even in alkaline solutions. Protamins, histons, and the usual proteins 

 form a series, therefore, in which the basic reaction is less and less marked. 

 The best known of the histons is the globin obtained from hemoglobin ; an- 

 other form has been obtained from the nucleohiston in the white corpuscles, 

 from the spermatozoa of mackerel (scombron), codfish (gadushiston), sea- 

 urchin (arbacin), and frog (lotahiston). They do not occur free in the liquids 

 or tissues of the body, but in combination, as in the case of hemoglobin. 

 They give the biuret reaction, a faint Millon reaction, and also respond to the 

 tests for sulphur. The products obtained by their hydrolytic cleavage are 

 much more numerous than in the case of the protamins a fact which would 

 indicate that their molecular structure is correspondingly more complex. 



The Compound Proteins. The chromoproteins may be defined as 

 consisting of a simple protein in combination with a pigment grouping, such 

 as occurs in the case of hemoglobin. A. number of such compounds are 

 known hemoglobin, hemocyanin, hemerythrin, chlorocruorin all char- 

 acterized physiologically by the fact that they serve to transport oxygen 

 from the air or water to the tissues. On boiling, heating with alkalies or 

 acids, etc., they readily decompose into their constituent parts (see Blood). 

 Glucoproteins are compounds of a carbohydrate group with a simple protein. 

 Numerous bodies have been put in this class ; some of them contain phos- 

 phorus (phosphoglucoproteins). Those free from phosphorus fall into two 

 divisions : one, the mucins, which on decomposition yield the carbohydrate 

 group in the form of an amino-sugar (glucosamin), and one, the chondropro- 

 teins, found in the connective tissues and in the pathological substance known 

 as amyloid, which yield their carbohydrate group in the form of chondroitin- 

 sulphuric acid (C 18 H., 7 NSO 17 ). True mucin is obtained from the secretion 

 of the salivary glands and the mucous glands of the various mucous mem- 

 branes. The nucleoproteins constitute the most interesting of the group 

 of compound proteins. They are recognized as forming an important con- 

 stituent of the cell nuclei. They may be defined as consisting of a compound 

 of simple protein with a nucleic acid. In the nuclei (head) of spermatozoa 

 the compound, in some cases at least (fishes), contains a nucleic acid and a 

 protamin. In other cases the protein constituent is more complex. On 

 digestion with pepsin-hydrochloric acid the more complex nucleoproteins 

 split, with the formation, first, of a protein substance and a simpler nucleo- 

 protein, richer in phosphorus and designated as a nuclein. On further de- 

 composition this latter yields a nucleic acid. Nucleic acid is, therefore, the 

 characteristic constituent, and a number of different forms have been de- 

 scribed, all rich in phosphorus, such as thymonucleic acid, salmonnucleic acid, 

 guanylic acid, etc. On hydrolytic decomposition they yield some of the 

 purin bases xanthin, guanin, adenin, etc.; some pyrimidin derivatives 

 uracil, thymin, cytosin; a carbohydrate group pentose, levulinic acid; and 

 phosphoric acid. These final decomposition products are characteristic of 

 the true nucleoproteins as distinguished from the phosphorus-containing 

 simple proteins, the nucleo-albumins or phosphoproteins, such as casein. 

 The percentage of phosphorus in the nucleoproteins varies, according to the 

 complexity of the molecule, between 0.5 and 1.6 per cent. 



The Albuminoids. This general name is reserved for a group of nitrog- 

 enous bodies found chiefly in the supporting connective tissues of the body, 

 such as the keratin of the epidermis, hairs, etc.; the elastin of the elastic 

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